The document was prepared using best effort. The authors make no warranty of any kind and shall not be liable in any event for incidental or consequential damages in connection with the application of the document.
Management Summary This report summarizes the results of the hardware assessment in the form of a Failure Modes, Effects, and Diagnostic Analysis (FMEDA) of the AXR Two-wire Magnetic Flowmeter, hardware revision Rev. 0 and software revision (SW1: Rev. 2.01, SW2: Rev.3.1). A Failure Modes, Effects, and Diagnostic Analysis is one of the steps to be taken to achieve functional safety certification per IEC 61508 of a device. From the FMEDA, failure rates are determined. The FMEDA that is described in this report concerns only the hardware of the AXR Flowmeter. For full functional safety certification purposes, all requirements of IEC 61508 must be considered.
AXR Flowmeter description: safety function, inputs, safety outputs, diagnostic capability, any external diagnostics that are required.
Table 1 gives an overview of the product that was considered in the FMEDA of the AXR Flowmeter.
Table 1 Version Overview
AXR Flowmeter AXR Two-wire Magnetic Flowmeter
The AXR Flowmeter is classified as a Type B1 element according to IEC 61508, having a hardware fault tolerance of 0.
The analysis shows that the AXR Flowmeter has a Safe Failure Fraction between 90% and 99% (assuming that the logic solver is programmed to detect over-scale and under-scale currents) and therefore meets hardware architectural constraints for up to SIL 2 as a single device.
Based on the assumptions listed in 4.3, the failure rates for the AXR Flowmeter are listed in section 4.4.
These failure rates are valid for the useful lifetime of the product, see Appendix A.
The failure rates listed in this report are based on over 350 billion unit operating hours of process industry field failure data. The failure rate predictions reflect realistic failures and include site specific failures due to human events for the specified Site Safety Index (SSI), see section 4.2.2.
A user of the AXR Flowmeter can utilize these failure rates in a probabilistic model of a safety instrumented function (SIF) to determine suitability in part for safety instrumented system (SIS) usage in a particular safety integrity level (SIL).
1 Type B element: “Complex” element (using micro controllers or programmable logic); for details see 7.4.4.1.3 of IEC 61508-2, ed2, 2010.
6 Terms and Definitions .............................................................................................. 14 7 Status of the Document ........................................................................................... 15
7.1 Liability ......................................................................................................................... 15 7.2 Version History ............................................................................................................. 15 7.3 Future enhancements .................................................................................................. 16 7.4 Release signatures ...................................................................................................... 16
Appendix A Lifetime of Critical Components .............................................................. 17 Appendix B Proof Tests to Reveal Dangerous Undetected Faults ............................. 18
B.1 Suggested Proof Test .................................................................................................. 18 Appendix C exida Environmental Profiles .............................................................. 19 Appendix D Determining Safety Integrity Level ....................................................... 20 Appendix E Site Safety Index ..................................................................................... 24
E.1 Site Safety Index Profiles ............................................................................................. 24 E.2 Site Safety Index Failure Rates – AXR Flowmeter ...................................................... 24
1 Purpose and Scope This document shall describe the results of the hardware assessment in the form of the Failure Modes, Effects and Diagnostic Analysis carried out on the AXR Flowmeter. From this, failure rates for each failure mode/category, useful life, and proof test coverage are determined.
The information in this report can be used to evaluate whether an element meets the average Probability of Failure on Demand (PFDAVG) requirements and if applicable, the architectural constraints / minimum hardware fault tolerance requirements per IEC 61508 / IEC 61511.
A FMEDA is part of the effort needed to achieve full certification per IEC 61508 or other relevant functional safety standard.
exida is one of the world’s leading accredited Certification Bodies and knowledge companies specializing in automation system safety, availability, and cybersecurity with over 500 person years of cumulative experience in functional safety, alarm management, and cybersecurity. Founded by several of the world’s top reliability and safety experts from manufacturers, operators and assessment organizations, exida is a global corporation with offices around the world. exida offers training, coaching, project oriented consulting services, safety engineering tools, detailed product assurance and ANSI accredited functional safety and cybersecurity certification. exida maintains a comprehensive failure rate and failure mode database on electronic and mechanical equipment and a comprehensive database on solutions to meet safety standards such as IEC 61508.
2.2 Roles of the parties involved
Yokogawa Electric Corporation Manufacturer of the AXR Flowmeter
exida Performed the hardware assessment
Yokogawa Electric Corporation contracted exida in February 2020 with the hardware assessment of the above-mentioned device.
2.3 Standards and literature used
The services delivered by exida were performed based on the following standards / literature.
[N1] IEC 61508-2: ed2, 2010 Functional Safety of Electrical/Electronic/Programmable Electronic Safety-Related Systems
[N9] Random versus Systematic – Issues and Solutions, September 2016
Goble, W.M., Bukowski, J.V., and Stewart, L.L., Random versus Systematic – Issues and Solutions, exida White Paper, PA: Sellersville, www.exida.com/resources/whitepapers, September 2016.
[N10] Assessing Safety Culture via the Site Safety IndexTM, April 2016
Bukowski, J.V. and Chastain-Knight, D., Assessing Safety Culture via the Site Safety IndexTM, Proceedings of the AIChE 12th Global Congress on Process Safety, GCPS2016, TX: Houston, April 2016.
[N11] Quantifying the Impacts of Human Factors on Functional Safety, April 2016
Bukowski, J.V. and Stewart, L.L., Quantifying the Impacts of Human Factors on Functional Safety, Proceedings of the 12th Global Congress on Process Safety, AIChE 2016 Spring Meeting, NY: New York, April 2016.
[N12] Criteria for the Application of IEC 61508:2010 Route 2H, December 2016
Criteria for the Application of IEC 61508:2010 Route 2H, exida White Paper, PA: Sellersville, www.exida.com, December 2016.
[N13] Using a Failure Modes, Effects and Diagnostic Analysis (FMEDA) to Measure Diagnostic Coverage in Programmable Electronic Systems, November 1999
Goble, W.M. and Brombacher, A.C., Using a Failure Modes, Effects and Diagnostic Analysis (FMEDA) to Measure Diagnostic Coverage in Programmable Electronic Systems, Reliability Engineering and System Safety, Vol. 66, No. 2, November 1999.
[N14] FMEDA – Accurate Product Failure Metrics, June 2015
Grebe, J. and Goble W.M., FMEDA – Accurate Product Failure Metrics, www.exida.com, June 2015.
2.4 exida tools used
[T1] FMEDAx 2.0.0.23856 exida FMEDA Tool
[T2] V4.3.0.4922 exSILentia
2.5 Reference documents
2.5.1 Documentation provided by Yokogawa Electric Corporation
[D1] IM 01E30D01-01EN, 7th
Edition, May 2017 AXR Two-wire Magnetic Flowmeter Instruction Manual
[D2] GS 01E30D01-01EN, 9th
Edition, January 10, 2018 AXR Two-wire Magnetic Flowmeter General Specifications
3 Product Description The AXR Flowmeter is a two-wire device that uses excitation coils and electrodes to measure fluid flow. HART or BRAIN communication signals are superimposed on the 4-20mA signal. Diagnostics include monitoring electrodes for the adherence of insulating material that would affect the flow measurement and a reverse calculation of the process variable.
The FMEDA includes the coils, electrodes and the two wire powered electronics. See diagram below.
.
Figure 1 AXR Flowmeter, Parts included in the FMEDA
Table 2 gives an overview of the product that was considered in the FMEDA of the AXR Flowmeter.
Table 2 Version Overview
AXR Flowmeter AXR Two-wire Magnetic Flowmeter
The AXR Flowmeter is classified as a Type B2 element according to IEC 61508, having a hardware fault tolerance of 0.
2 Type B element: “Complex” element (using micro controllers or programmable logic); for details see 7.4.4.1.3 of IEC 61508-2, ed2, 2010.
4 Failure Modes, Effects, and Diagnostic Analysis The Failure Modes, Effects, and Diagnostic Analysis was performed based on the documentation in section 2.5.1 and is documented in [R1] to [R2].
When the effect of a certain failure mode could not be analyzed theoretically, the failure modes were introduced on component level and the effects of these failure modes were examined on system level, see Fault Injection Test Report [D8].
4.1 Failure categories description
In order to judge the failure behavior of the AXR Flowmeter, the following definitions for the failure of the device were considered.
Fail-Safe State Failure that deviates the process signal or the actual output by more than 2% of span, drifts toward the user defined threshold (Trip Point) and that leaves the output within the active scale.
Fail Safe Failure that causes the device to go to the defined fail-safe state without a demand from the process.
Fail Detected Failure that causes the output signal to go to the predefined alarm state (below 3.6 mA or above 21.0 mA).
Fail Dangerous Failure that deviates the process signal or the actual output by more than 2% of span, drifts away from the user defined threshold (Trip Point) and that leaves the output within the active scale.
Fail Dangerous Undetected Failure that is dangerous and that is not being diagnosed by automatic diagnostics.
Fail Dangerous Detected Failure that is dangerous but is detected by automatic diagnostics.
Fail High Failure that causes the output signal to go to the over-range or high alarm output current (> 21 mA).
Fail Low Failure that causes the output signal to go to the under-range or low alarm output current (< 3.6 mA).
No Effect Failure of a component that is part of the safety function but that has no effect on the safety function.
Annunciation Detected Failure that does not directly impact safety but does impact the ability to detect a future fault (such as a fault in a diagnostic circuit) and that is detected by internal diagnostics. A Fail Annunciation Detected failure leads to a false diagnostic alarm.
Annunciation Undetected Failure that does not directly impact safety but does impact the ability to detect a future fault (such as a fault in a diagnostic circuit) and that is not detected by internal diagnostics.
The failure categories listed above expand on the categories listed in IEC 61508 in order to provide a complete set of data needed for design optimization.
Depending on the application, a Fail High or a Fail Low failure can either be safe or dangerous and may be detected or undetected depending on the programming of the logic solver. Consequently, during a Safety Integrity Level (SIL) verification assessment the Fail High and Fail Low failure categories need to be classified as safe or dangerous, detected or undetected.
The Annunciation failures are provided for those who wish to do reliability modeling more detailed than required by IEC61508. It is assumed that the probability model will correctly account for the Annunciation failures.
A FMEDA (Failure Mode Effect and Diagnostic Analysis) is a failure rate prediction technique based on a study of design strength versus operational profile stress. It combines design FMEA techniques with extensions to identify automatic diagnostic techniques and the failure modes relevant to safety instrumented system design. It is a technique recommended to generate failure rates for each failure mode category [N13, N14].
4.2.2 Failure rates
The accuracy of any FMEDA analysis depends upon the component reliability data as input to the process. Component data from consumer, transportation, military or telephone applications could generate failure rate data unsuitable for the process industries. The component data used by exida in this FMEDA is from the Electrical and Mechanical Component Reliability Handbooks [N3] which were derived using over 350 billion unit operational hours of process industry field failure data from multiple sources and failure data formulas from international standards. The component failure rates are provided for each applicable operational profile and application, see Appendix C. The exida profile chosen for this FMEDA was < Profile 2 > as this was judged to be the best fit for the product and application information submitted by Yokogawa Electric Corporation. It is expected that the actual number of field failures will be less than the number predicted by these failure rates.
Early life failures (infant mortality) are not included in the failure rate prediction as it is assumed that some level of commission testing is done. End of life failures are not included in the failure rate prediction as useful life is specified.
The failure rates are predicted for a Site Safety Index of SSI=2 [N10, N11] as this level of operation is common in the process industries. Failure rate predictions for other SSI levels are included in the exSILentia® tool from exida.
The user of these numbers is responsible for determining the failure rate applicability to any particular environment. exida Environmental Profiles listing expected stress levels can be found in Appendix C. Some industrial plant sites have high levels of stress. Under those conditions the failure rate data is adjusted to a higher value to account for the specific conditions of the plant. exida has detailed models available to make customized failure rate predictions. Contact exida.
Accurate plant specific data may be used to check validity of this failure rate data. If a user has data collected from a good proof test reporting system such as exida SILStatTM that indicates higher failure rates, the higher numbers shall be used.
4.3 Assumptions
The following assumptions have been made during the Failure Modes, Effects, and Diagnostic Analysis of the AXR Flowmeter.
The worst-case assumption of a series system is made. Therefore, only a single component failure will fail the entire AXR Flowmeter and propagation of failures is not relevant.
Failure rates are constant for the useful life period.
Any product component that cannot influence the safety function (feedback immune) is excluded. All components that are part of the safety function including those needed for normal operation are included in the analysis.
The stress levels are specified in the exida Profile used for the analysis are limited by the manufacturer’s published ratings.
Practical fault insertion tests have been used when applicable to demonstrate the correctness of the FMEDA results.
The HART protocol is only used for setup, calibration, and diagnostics purposes, not for safety critical operation.
The application program in the logic solver is constructed in such a way that Fail High and Fail Low failures are detected regardless of the effect, safe or dangerous, on the safety function.
Materials are compatible with process conditions.
The device is installed and operated per manufacturer’s instructions.
External power supply failure rates are not included.
Recommended calibration intervals and replacement schedules of the electrochemical cartridge are observed and used to implement frequent proof testing of the device.
Worst-case internal fault detection time is 1 hour.
4.4 Results
Using reliability data extracted from the exida Electrical and Mechanical Component Reliability Handbook the following failure rates resulted from the AXR Flowmeter FMEDA.
Table 3 lists the failure rates for the AXR Flowmeter with a Site Safety Index (SSI) of 2 (good site maintenance practices). See Appendix E for an explanation of SSI and the failure rates for SSI of 4 (ideal maintenance practices).
Table 3 Failure rates for Static Applications3 with Good Maintenance Assumptions in FIT @ SSI=2
Failure Category Failure Rate (FIT)
Fail Safe Undetected 42
Fail Dangerous Detected 528
Fail Detected (detected by internal diagnostics) 486
Fail High (detected by logic solver) 20
Fail Low (detected by logic solver) 22
Fail Dangerous Undetected 69
No Effect 234
Annunciation Detected 87
Annunciation Undetected 98
3 Static Application failure rates are applicable if the device is static for a period of more than 200 hours.
Table 4 lists the failure rates for the AXR Flowmeter according to IEC 61508.
Table 4 Failure rates for Static Applications with Good Maintenance Assumptions in FIT @ SSI=2 according to IEC 61508
Device λSD λSU4 λDD λDU # SFF
AXR Two-wire Magnetic Flowmeter 20 42 528 69 234 90.8%
Where:
λSD = Fail Safe Detected
λSU = Fail Safe Undetected
λDD = Fail Dangerous Detected
λDU = Fail Dangerous Undetected
# = No Effect Failures
These failure rates are valid for the useful lifetime of the product, see Appendix A.
According to IEC 61508-2 the architectural constraints of an element must be determined. This can be done by following the 1H approach according to 7.4.4.2 of IEC 61508-2 or the 2H approach according to 7.4.4.3 of IEC 61508-2, or the approach according to IEC 61511:2016 which is based on 2H (see Section 5.2).
The 1H approach involves calculating the Safe Failure Fraction for the entire element.
The 2H approach involves assessment of the reliability data for the entire element according to 7.4.4.3.3 of IEC 61508-2.
The analysis shows that the AXR Flowmeter has a Safe Failure Fraction between 90% and 99% (assuming that the logic solver is programmed to detect over-scale and under-scale currents) and therefore meets hardware architectural constraints for up to SIL 2 as a single device..
Table 9 lists the failure rates for the AXR Flowmeter according to IEC 61508 with a Site Safety Index (SSI) of 4 (perfect site maintenance practices). This data should not be used for SIL verification and is provided only for comparison with other analysis than has assumed perfect maintenance. See Appendix E for an explanation of SSI.
4 It is important to realize that the No Effect failures are no longer included in the Safe Undetected failure category according to IEC 61508, ed2, 2010.
5 Using the FMEDA Results The following section(s) describe how to apply the results of the FMEDA.
5.1 PFDavg calculation AXR Flowmeter
Using the failure rate data displayed in section 4.4, and the failure rate data for the associated element devices, an average the Probability of Failure on Demand (PFDavg) calculation can be performed for the element.
Probability of Failure on Demand (PFDavg) calculation uses several parameters, many of which are determined by the particular application and the operational policies of each site. Some parameters are product specific and the responsibility of the manufacturer. Those manufacturer specific parameters are given in this third-party report.
Probability of Failure on Demand (PFDavg) calculation is the responsibility of the owner/operator of a process and is often delegated to the SIF designer. Product manufacturers can only provide a PFDavg by making many assumptions about the application and operational policies of a site. Therefore, use of these numbers requires complete knowledge of the assumptions and a match with the actual application and site.
Probability of Failure on Demand (PFDavg) calculation is best accomplished with exida’s exSILentia tool. See Appendix D for a complete description of how to determine the Safety Integrity Level for an element. The mission time used for the calculation depends on the PFDavg target and the useful life of the product. The failure rates and the proof test coverage for the element are required to perform the PFDavg calculation. The proof test coverage for the suggested proof test are listed in Table 6.
5.2 exida Route 2H Criteria
IEC 61508, ed2, 2010 describes the Route 2H alternative to Route 1H architectural constraints. The standard states:
"based on data collected in accordance with published standards (e.g., IEC 60300-3-2: or ISO 14224); and, be evaluated according to
the amount of field feedback; and
the exercise of expert judgment; and when needed
the undertake of specific tests,
in order to estimate the average and the uncertainty level (e.g., the 90% confidence interval or the probability distribution) of each reliability parameter (e.g., failure rate) used in the calculations."
exida has interpreted this to mean not just a simple 90% confidence level in the uncertainty analysis, but a high confidence level in the entire data collection process. As IEC 61508, ed2, 2010 does not give detailed criteria for Route 2H, exida has established the following:
1. field unit operational hours of 100,000,000 per each component; and
2. a device and all its components have been installed in the field for one year or more; and
3. operational hours are counted only when the data collection process has been audited for correctness and completeness; and
4. failure definitions, especially "random" vs. "systematic" [N9] are checked by exida; and
5. every component used in an FMEDA meets the above criteria.
This set of requirements is chosen to assure high integrity failure data suitable for safety integrity verification. [N12}
6 Terms and Definitions Automatic Diagnostics Tests performed online internally by the device or, if specified,
externally by another device without manual intervention.
exida criteria A conservative approach to arriving at failure rates suitable for use in hardware evaluations utilizing the 2H Route in IEC 61508-2.
Fault tolerance Ability of a functional unit to continue to perform a required function in the presence of faults or errors (IEC 61508-4, 3.6.3).
FIT Failure in Time (1x10-9 failures per hour)
FMEDA Failure Mode Effect and Diagnostic Analysis
HFT Hardware Fault Tolerance
PFDavg Average Probability of Failure on Demand
SFF Safe Failure Fraction, summarizes the fraction of failures which lead to a safe state plus the fraction of failures which will be detected by automatic diagnostic measures and lead to a defined safety action.
SIF Safety Instrumented Function
SIL Safety Integrity Level
SIS Safety Instrumented System – Implementation of one or more Safety Instrumented Functions. A SIS is composed of any combination of sensor(s), logic solver(s), and final element(s).
Type A element “Non-Complex” element (using discrete components); for details see 7.4.4.1.2 of IEC 61508-2
Type B element “Complex” element (using complex components such as micro controllers or programmable logic); for details see 7.4.4.1.3 of IEC 61508-2
exida prepares FMEDA reports based on methods advocated in International standards. Failure rates are obtained from a collection of industrial databases. exida accepts no liability whatsoever for the use of these numbers or for the correctness of the standards on which the general calculation methods are based.
Due to future potential changes in the standards, product design changes, best available information and best practices, the current FMEDA results presented in this report may not be fully consistent with results that would be presented for the identical model number product at some future time. As a leader in the functional safety market place, exida is actively involved in evolving best practices prior to official release of updated standards so that our reports effectively anticipate any known changes. In addition, most changes are anticipated to be incremental in nature and results reported within the previous three-year period should be sufficient for current usage without significant question.
Most products also tend to undergo incremental changes over time. If an exida FMEDA has not been updated within the last three years, contact the product vendor to verify the current validity of the results.
7.2 Version History
Contract Number
Report Number Revision Notes
Q10/06-091 YOK 10-06-091 R001 V0R1 FMEDA AXR.docx
Draft; Date 12 August 2010
Q10/06-091 YOK 10-06-091 R001 V1R1 FMEDA AXR.docx
Released to Yokogawa Electric Corporation; 13 August 2010
Q20/02-144 YOK 10-06-091 R001 V2R1 FMEDA AXR.docx
Update FMEDA on IEC 61508-2: ed2, 2010 2, 9 March 2020
Q20/02-144 YOK 10-06-091 R001 V2R2 FMEDA AXR.docx
Draft for Customer Review; 10 March 2020
Q20/02-144 YOK 10-06-091 R001 V2R3 FMEDA AXR.docx
Update Release on IEC 61508-2: ed2, 2010; 12 March 2020
Reviewer: V0, R1: William Goble (exida); 13 August 2010
V2, R1: Kaoru Sonoda (exida-Japan); 9 March 2020
Status: Update Release on IEC 61508-2: ed2, 12 March 2020
Appendix A Lifetime of Critical Components According to section 7.4.9.5 of IEC 61508-2, a useful lifetime, based on experience, should be determined and used to replace equipment before the end of useful life.
Although a constant failure rate is assumed by the exida FMEDA prediction method (see section 4.2.2) this only applies provided that the useful lifetime5 of components is not exceeded. Beyond their useful lifetime, the result of the probabilistic calculation method is likely optimistic, as the probability of failure significantly increases with time. The useful lifetime is highly dependent on the subsystem itself and its operating conditions.
Table 5 shows which components are contributing to the dangerous undetected failure rate and therefore to the PFDavg calculation and what their estimated useful lifetime is.
Table 5 Useful lifetime of components contributing to dangerous undetected failure rate
It is the responsibility of the end user to maintain and operate the AXR Flowmeter per manufacturer’s instructions. Furthermore, regular inspection should show that all components are clean and free from damage.
When plant experience indicates a shorter useful lifetime than indicated in this appendix, the number based on plant experience should be used.
5 Useful lifetime is a reliability engineering term that describes the operational time interval where the failure rate of a device is relatively constant. It is not a term which covers product obsolescence, warranty, or other commercial issues.
Appendix B Proof Tests to Reveal Dangerous Undetected Faults According to section 7.4.5.2 f) of IEC 61508-2 proof tests shall be undertaken to reveal dangerous faults which are undetected by automatic diagnostic tests. This means that it is necessary to specify how dangerous undetected faults which have been noted during the Failure Modes, Effects, and Diagnostic Analysis can be detected during proof testing.
B.1 Suggested Proof Test
The suggested proof test described in Table 6 will detect 78% of possible DU failures in the AXR Flowmeter.
The suggested the AXR Flowmeter proof test consists of a setting the output to the min and max, and a calibration check, see Table 6.
Table 6 Suggested Proof Test – HART Communications
Step Action
1. Bypass the safety function and take appropriate action to avoid a false trip
2. Use HART communications to retrieve any diagnostics and take appropriate action.
3. Send a HART command to the flowmeter to go to the high alarm current output and verify that the analog current reaches that value6.
4. Send a HART command to the flowmeter to go to the low alarm current output and verify that the analog current reaches that value7.
5. Perform a two-point calibration8 of the flowmeter over the full working range.
6. Check current output when there is no flow in the meter9.
7. Check current output when there is a typical flow in the meter.
8. Remove the bypass and otherwise restore normal operation
6 This tests for compliance voltage problems such as a low loop power supply voltage or increased wiring resistance. This also tests for other possible failures. 7 This tests for possible quiescent current related failures. 8 If the two-point calibration is performed with electrical instrumentation, this step of the proof test will not detect any failures of the coils and electrodes. 9 Checking the current output at no flow and a typical flow rate allows some coil and electrode failure detection.
13 Humidity rating per IEC 60068-2-3 14 Shock rating per IEC 60068-2-27 15 Vibration rating per IEC 60068-2-6 16 Chemical Corrosion rating per ISA 71.04 17 Surge rating per IEC 61000-4-5 18 EMI Susceptibility rating per IEC 61000-4-3 19 ESD (Air) rating per IEC 61000-4-2
Appendix D Determining Safety Integrity Level The information in this appendix is intended to provide the method of determining the Safety Integrity Level (SIL) of a Safety Instrumented Function (SIF). The numbers used in the examples are not for the product described in this report.
Three things must be checked when verifying that a given Safety Instrumented Function (SIF) design meets a Safety Integrity Level (SIL) [N4] and [N7].
These are:
A. Systematic Capability or Prior Use Justification for each device meets the SIL level of the SIF;
B. Architecture Constraints (minimum redundancy requirements) are met; and
C. a PFDavg calculation result is within the range of numbers given for the SIL level.
A. Systematic Capability (SC) is defined in IEC61508:2010. The SC rating is a measure of design quality based upon the methods and techniques used to design and development a product. All devices in a SIF must have a SC rating equal or greater than the SIL level of the SIF. For example, a SIF is designed to meet SIL 3 with three pressure transmitters in a 2oo3 voting scheme. The transmitters have an SC2 rating. The design does not meet SIL 3. Alternatively, IEC 61511 allows the end user to perform a "Prior Use" justification. The end user evaluates the equipment to a given SIL level, documents the evaluation and takes responsibility for the justification.
B. Architecture constraints require certain minimum levels of redundancy. Different tables show different levels of redundancy for each SIL level. A table is chosen and redundancy is incorporated into the design [N8].
C. Probability of Failure on Demand (PFDavg) calculation uses several parameters, many of which are determined by the particular application and the operational policies of each site. Some parameters are product specific and the responsibility of the manufacturer. Those manufacturer specific parameters are given in this third-party report.
A Probability of Failure on Demand (PFDavg) calculation must be done based on a number of variables including:
1. Failure rates of each product in the design including failure modes and any diagnostic coverage from automatic diagnostics (an attribute of the product given by this FMEDA report); 2. Redundancy of devices including common cause failures (an attribute of the SIF design); 3. Proof Test Intervals (assignable by end user practices); 4. Mean Time to Restore (an attribute of end user practices); 5. Proof Test Effectiveness; (an attribute of the proof test method used by the end user with an example given by this report); 6. Mission Time (an attribute of end user practices); 7. Proof Testing with process online or shutdown (an attribute of end user practices); 8. Proof Test Duration (an attribute of end user practices); and 9. Operational/Maintenance Capability (an attribute of end user practices).
The product manufacturer is responsible for the first variable. Most manufacturers use the exida FMEDA technique which is based on over 350 billion hours of field failure data in the process industries to predict these failure rates as seen in this report. A system designer chooses the second variable. All other variables are the responsibility of the end user site. The exSILentia® SILVerTM software considers all these variables and provides an effective means to calculate PFDavg for any given set of variables.
Simplified equations often account for only for first three variables. The equations published in IEC 61508-6, Annex B.3.2 [N1] cover only the first four variables. IEC61508-6 is only an informative portion of the standard and as such gives only concepts, examples and guidance based on the idealistic assumptions stated. These assumptions often result in optimistic PFDavg
calculations and have indicated SIL levels higher than reality. Therefore, idealistic equations should not be used for actual SIF design verification.
All the variables listed above are important. As an example, consider a high-level protection SIF. The proposed design has a single SIL 3 certified level transmitter, a SIL 3 certified safety logic solver, and a single remote actuated valve consisting of a certified solenoid valve, certified scotch yoke actuator and a certified ball valve. Note that the numbers chosen are only an example and not the product described in this report.
Using exSILentia with the following variables selected to represent results from simplified equations:
Mission Time = 5 years Proof Test Interval = 1 year for the sensor and final element, 5 years for the logic solver Proof Test Coverage = 100% (ideal and unrealistic but commonly assumed) Proof Test done with process offline
This results in a PFDavg of 6.82E-03 which meets SIL 2 with a risk reduction factor of 147. The subsystem PFDavg contributions are Sensor PFDavg = 5.55E-04, Logic Solver PFDavg = 9.55E-06, and Final Element PFDavg = 6.26E-03. See Figure 2.
Figure 2: exSILentia results for idealistic variables. If the Proof Test Interval for the sensor and final element is increased in one year increments, the results are shown in Figure 3.
If a set of realistic variables for the same SIF are entered into the exSILentia software including:
Mission Time = 25 years Proof Test Interval = 1 year for the sensor and final element, 5 years for the logic solver Proof Test Coverage = 90% for the sensor and 70% for the final element Proof Test Duration = 2 hours with process online. MTTR = 48 hours Maintenance Capability = Medium for sensor and final element, Good for logic solver
with all other variables remaining the same, the PFDavg for the SIF equals 5.76E-02 which barely meets SIL 1 with a risk reduction factor 17. The subsystem PFDavg contributions are Sensor PFDavg = 2.77E-03, Logic Solver PFDavg = 1.14E-05, and Final Element PFDavg = 5.49E-02 (Figure 4).
Appendix E Site Safety Index Numerous field failure studies have shown that the failure rate for a specific device (same Manufacturer and Model number) will vary from site to site. The Site Safety Index (SSI) was created to account for these failure rates differences as well as other variables. The information in this appendix is intended to provide an overview of the Site Safety Index (SSI) model used by exida to compensate for site variables including device failure rates.
E.1 Site Safety Index Profiles
The SSI is a number from 0 – 4 which is an indication of the level of site activities and practices that contribute to the safety performance of SIF’s on the site. Table 8 details the interpretation of each SSI level. Note that the levels mirror the levels of SIL assignment and that SSI 4 implies that all requirements of IEC 61508 and IEC 61511 are met at the site and therefore there is no degradation in safety performance due to any end-user activities or practices, i.e., that the product inherent safety performance is achieved. Several factors have been identified thus far which impact the Site Safety Index (SSI). These include the quality of: Commission Test Safety Validation Test Proof Test Procedures Proof Test Documentation Failure Diagnostic and Repair Procedures Device Useful Life Tracking and Replacement Process SIS Modification Procedures SIS Decommissioning Procedures and others
Table 8 exida Site Safety Index Profiles
Level Description
SSI 4
Perfect - Repairs are always correctly performed, Testing is always done correctly and on schedule, equipment is always replaced before end of useful life, equipment is always selected according to the specified environmental limits and process compatible materials. Electrical power supplies are clean of transients and isolated, pneumatic supplies and hydraulic fluids are always kept clean, etc. Note: This level is generally considered not possible but retained in the model for comparison purposes.
SSI 3
Almost perfect - Repairs are correctly performed, Testing is done correctly and on schedule, equipment is normally selected based on the specified environmental limits and a good analysis of the process chemistry and compatible materials. Electrical power supplies are normally clean of transients and isolated, pneumatic supplies and hydraulic fluids are mostly kept clean, etc. Equipment is replaced before end of useful life, etc.
SSI 2 Good - Repairs are usually correctly performed, Testing is done correctly and mostly on schedule, most equipment is replaced before end of useful life, etc.
SSI 1 Medium – Many repairs are correctly performed, Testing is done and mostly on schedule, some equipment is replaced before end of useful life, etc.
SSI 0 None - Repairs are not always done, Testing is not done, equipment is not replaced until failure, etc.
E.2 Site Safety Index Failure Rates – AXR Flowmeter
Failure rates of each individual device in the SIF are increased or decreased by a specific multiplier which is determined by the SSI value and the device itself. It is known that final elements are more likely to be negatively impacted by less than ideal end-user practices than are sensors or logic solvers. By increasing or decreasing device failure rates on an individual
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